A protein can be investigated from various aspects with high precision when the protein purified to a purity based on a three-dimensional structure can be obtained. In general, however, a large amount of labor is necessary for purification of protein to such level. Particularly, a highly pure protein, which is rich in active conformation, not only having a high purity on an SDS gel (purity based on a primary structure), but also having a high purity based on a three-dimensional structure is extremely difficult to obtain. As a method meeting this object, purification using an affinity resin with an immobilized ligand having a specific binding ability has been employed. In the prior art, however, a ligand compound or salt is used at a high concentration to release and elute out a protein bound to a ligand on a resin. When a low-molecular-weight compound such as a pharmaceutical product and the like is used as a ligand, since solubility thereof in water is generally insufficient, a solution having a sufficient concentration to release the protein often cannot be prepared. Even when a high concentration ligand solution could be prepared, denaturation of protein due to the high concentration of ligand is problematic. Moreover, even when a high concentration salt is used, since the function is theoretically based on the antagonism to a nonspecific hydrogen bond due to the high concentration salt, it is necessary to add a salt at so high a concentration as to forcibly disrupt a stable ligand-protein complex structure under specific binding. In this case, a problem of denaturation of protein occurs, which is caused by simultaneous cleavage of a hydrogen bond necessary for maintaining the three-dimensional structure of the protein.
It is an object of the present invention to provide a method that enables selective purification of a protein to a high purity. More specifically, the present invention aims at providing a technique that enables, without using a high concentration ligand compound and salt, release and elution of a protein from an affinity resin.